1. Field of the Invention
The present invention relates to optical interleavers for combining and separating pluralities of optical wavelength channels. More particularly, this invention pertains to apparatus for assuring alignment of the optical channels at the output ports of an interleaver to pre-designated International Telecommunications Union (ITU) grid frequencies.
2. Description of the Prior Art
The interleaver is a device that functions in an optical network to combine two input sets of wavelengths in which the channels of one set of wavelengths are offset by one half the channel spacing from those of the other set. Such a device is ideal for ultra dense networks. Further, interleavers can work in reverse to separate a single densely packed channel set into two output fibers, each of twice the channel spacing of the original set. They may be cascaded to provide further channel separation on four output fibers, each transmitting one fourth of the number of channels and four times the channel spacing. An interleaver or an array of interleavers allows the use of simpler thin-film filters or arrayed waveguide gratings to separate the individual channels.
FIG. 1 is schematic view of an optical interleaver 10. Such a device operates upon an interferometric principle and the analysis that follows is applicable to a number of two-beam interferometric interleavers including Mach Zender and birefringent plate.
The interleaver 10 comprises a first coupler 12 for splitting light from a light source 14 into two beams. The beams travel separate optical paths 16 and 18 that terminate at a second coupler 20. Each of the couplers 12 and 20 is a fused biconical coupler made, as is well known in the art, in accordance with biconical tapered fusion technology. They may comprise a pair of optical fibers that have been stripped of their outer jackets and carefully cleaned. The claddings of the glass fibers are held in contact, heated to melting temperature and tension applied to reduce the thickness in the region of contact. At this point, the cores of the fibers (each about 9 microns in diameter) are very closely spaced to thereby achieve optical coupling between the two fiber cores. The resultant device is commonly encapsulated in a quartz tube. Through the phenomenon of evanescent coupling, light traveling through the core of one fiber is coupled into the core of the other fiber resulting in xe2x80x9csplittingxe2x80x9d of the optical signal. A coupler may act in reverse to combine the light traveling through the two fibers into a single fiber, thus acting as a xe2x80x9ccombinerxe2x80x9d. In FIG. 1, the coupler 12 is shown to act as a splitter while the coupler 20 acts as a combiner.
FIG. 2 is a plot of the interleaver""s frequency response, namely the normalized output power versus optical frequency (in THz) as defined by the preceding equations for an interferometer based upon the principle of the Mach-Zender interferometer having the following parameters: xcex1=0.51, xcex2=0.49 (power splitting ratios); L1=0.25 dB, L2=1.0 dB (optical power loss of paths 16, 18 in dB, related to amplitude loss coefficient xcex4 by exe2x88x922xcex4=10xe2x88x92L/10); optical path length difference=1.5 mm. The optical path length difference xcex94xcex8 is related to optical phase shift difference xcex94"PHgr" by
xcex94"PHgr"=2Πxcexdxcex94xcex8/cxe2x80x83xe2x80x83(9) 
Where c is the speed of light in a vacuum and xcex94xcex8 is related to the refractive index and length L as xcex94xcex8=xcex94(nL).
The logarithmic plot of FIG. 2 with the output taken at port xe2x80x9c1xe2x80x9d of FIG. 1 indicated by the succession of maxima and minima of the curve denoted 22 and the output taken at port xe2x80x9c2xe2x80x9d of FIG. 1 indicated by the succession of maxima and minima of the curve denoted 24 illustrates a frequency spacing between two peaks of a given output of 0.2 THz (200 GHz) with the signals at the two outputs shifted with respect to one another by 0.1 THz (100 GHz). Since a non-zero loss is assumed, the peak amplitudes at the two outputs do not equal 1.0.
The operation of an interleaver as a multi-channel signal splitter can be understood from FIG. 2. Assuming that the input is a series of mutually incoherent wave channels whose frequency bands do not overlap and that are separated by 100 GHz, the interleaver 10 separates adjacent channels as follows: xe2x80x9coddxe2x80x9d frequency channels are forwarded to the output port 1 of FIG. 1 as constructive interference occurs at this output for such frequencies while xe2x80x9cevenxe2x80x9d frequency channels are forwarded to output port 2 as constructive interference for even channels occurs at output port 2.
Various issued and pending U.S. patents and patent applications address the critical relationship between channel spacing and optical path length difference in an interleaver operating as a Mach Zehnder interferometer. These include U.S. Pat. No. 6,031,948 of Chen covering xe2x80x9cFused-Fiber Multi-Window Wavelength Division Multiplexer Using an Unbalanced Mach-Zehnder Interferometer and Method of Making Samexe2x80x9d and pending U.S. patent application Ser. Nos. 09/861,910 and 09/862,146, now allowed, of Dent et al. covering xe2x80x9cMethod For Making All Fiber Interleaver With Continuous Fiber Armxe2x80x9d and xe2x80x9cOptical Interleaver With Image Transfer Elementxe2x80x9d respectively. Each of the pending applications is the property of the assignee herein.
In addition to providing interleaver designs characterized by optical path length differences that assure precise channel spacing, it is essential that optical channels remain tuned to the optical signal grid at all times and under all environmental conditions. (Generally, this will be the internationally-recognized ITU grid.) Tuning assures that the optical channels will be optimally processed by the elements of the system. Otherwise, power loss, crosstalk and signal distortion may be experienced.
Channel tuning requires maintenance and adjustment of the phases of the sinusoidal interleaver outputs. Such phase is known to be temperature sensitive, requiring the interleaver to be thermally controlled, a relatively difficult and expensive process.
The present invention overcomes the foregoing shortcomings of the prior art by providing, in a first aspect, an optical interleaver for separating an input DWDM signal occupying a nominal frequency band into two output signals. One of such output signals comprises a plurality of odd optical channels and the other comprises a plurality of even optical optical channels within the frequency band.
Such interleaver includes a first optical fiber and a second optical fiber, each having opposed ends. The first and second fibers are fused together intermediate their ends to form first and second couplers. In this way, an input section is defined before the first coupler, an interferometer section is defined between the first and second couplers, and an output section is defined after the second coupler.
A first tap coupler is provided within the input section for receiving a monitor signal having a nominal frequency. A second tap coupler is provided within the output section for providing an interleaver output signal. An optical path length adjustment element is provided for shifting the monitor signal in frequency space. Such optical path length adjustment element is responsive to a feedback signal.
A circuit is provided for generating the feedback signal. Such circuit is responsive to the interleaver output signal.
In a second aspect the invention provides a method for aligning the channels at an output of an optical interleaver to a set of nominal values within a nominal frequency band in frequency space. Such interleaver includes a first and a second optical fiber, each having opposed ends and fused together between their ends for form first and second couplers to define an input section before the first coupler, an interferometer section between the first and second couplers, and an output section after the second coupler. Such method is begun by inputting a monitor signal of nominal frequency outside said frequency band at said input section. The monitor signal is observed at the output section and at least one of the fibers is adjusted within the interferometer section in response to the observed monitor signal. Such process is continued as long as the center frequency approaches the nominal value.
The foregoing and additional features and advantages of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout both the written description and the drawings.